I haven't worked with Sintra (Closed-cell PVC) myself, but I'm told it is lovely for this type of armor. It lets you skip the whole hardening step. For those thin layers along the front of the pauldrons, you'll want the 1mm stuff.
As far as clay, most of the air dry stuff is gonna give you less detail. If you can rough sculpt quickly, you can use Apoxie sculpt, which is an epoxy based clay-like material. It doesn't shrink, and it can be sanded when cured to get higher detail.
For larger sections, you can get into Bondo putty (or other automotive body filler). You slap it on with a spatula in excess, and you sand it down to your desired shape. Then you use surfacing putty to fix the pinhole flaws that always show up. Then sand smooth, prime, and paint.
Wonderflex is often used for armor, but it is a pain to get this sort of detail out of it.